As described by Sullivan & Lynch in U.S. Pat. No. 5,199,424, issued on Apr. 6, 1993, the application of continuous positive airway pressure (CPAP) has been used as a means of treating the occurrence of obstructive sleep apnea. The patient is connected to a positive pressure air supply by means of a nose mask or nasal prongs. The air supply breathed by the patient is slightly greater than atmospheric pressure. It has been found that the application of continuous positive airway pressure provides what can be described as a “pneumatic splint”, supporting and stabilizing the upper airway and thus eliminating the occurrence of upper airway occlusions. It is effective in eliminating both snoring and obstructive sleep apnea and in many cases, is effective in treating central and mixed apnea.
In U.S. Pat. No. 5,549,106 to Gruenke, issued on Aug. 27, 1996, an apparatus is disclosed that is intended for facilitating the respiration of a patient for treating mixed and obstructive sleep apnea. The device increases nasal air pressure delivered to the patient's respiratory passages just prior to inhalation and by subsequently decreasing the pressure to ease exhalation effort.
In U.S. Pat. No. 5,245,995 Sullivan discusses how snoring and abnormal breathing patterns can be detected by inspiration and expiration pressure measurements while sleeping, thereby leading to early indication of preobstructive episodes or other forms of breathing disorder. Particularly, patterns of respiratory parameters are monitored, and CPAP pressure is raised on the detection of pre-defined patterns to provide increased airway pressure to, ideally, subvert the occurrence of the obstructive episodes and the other forms of breathing disorder.
As described by Berthon-Jones in U.S. Pat. No. 5,704,345, issued on Jan. 6, 1998, various techniques are known for sensing and detecting abnormal breathing patterns indicative of obstructed breathing. Berthon-Jones describes methods based on detecting events such as apnea, snoring, and respiratory flow flattening. Treatment pressure may be automatically adjusted in response to the detected conditions.
As described by Wickham in International Patent Application PCT/AU01/01948 (Publication No. WO0218002), a flow flattening determination may be further based upon different weighting factors. The weighing factors are applied to sections of the airflow to improve sensitivity to various types of respiration obstructions.
Other methods for detecting obstruction have also been used. For example, in U.S. Pat. Nos. 5,490,502 and 5,803,066, Rapport discloses a method and apparatus for optimizing the controlled positive pressure to minimize the flow of air from a flow generator while attempting to ensure that flow limitation in the patient's airway does not occur. Controlled positive pressure to the airway of a patient is adjusted by detecting flow limitation from the shape of an inspiratory flow waveform. The pressure setting is raised, lowered or maintained depending on whether flow limitation has been detected and on the previous actions taken by the system.
In U.S. Pat. No. 5,645,053, Remmers describes a system for automatically and continuously regulating the level of nasal pressure to an optimal value during OSA (Obstructive Sleep Apnea) treatment. Parameters related to the shape of a time profile of inspiratory flow are determined including a degree of roundness and flatness of the inspiratory profile. OSA therapy is then implemented by automatically re-evaluating an applied pressure and continually searching for a minimum pressure required to adequately distend a patient's pharyngeal airway.
Despite the availability of such devices for treating OSA, some sleep obstructive events may still go untreated with the use of some devices. Thus, new methods of automated detection and treatment of obstructive events may be desirable.